The components of an analog transmission chain all exhibit a greater or lesser degree of nonlinear behavior depending on the specific features of the transmitted signal. For example, it is difficult to provide a transmission chain with linear behavior over a wide bandwidth and a large signal power range.
Typical examples of sources of nonlinear distortion are digital/analog or analog/digital converters and amplifiers, in particular power amplifiers. This is because the latter operate as close as possible to saturation in order to maximize the added power efficiency and operating an amplifier close to its saturation point introduces nonlinear distortion.
In the case of a digitally modulated signal, the most negative effects of nonlinearity are an increase in the error vector magnitude (EVM) and the constellation points moving closer together. In order to increase spectral efficiency, there has been a change in the field of high spectral efficiency modulation to amplitude and phase shift keying (APSK) constellations which have plots which are more spaced apart than M-PSK (m-state phase shift keying) constellation plots but are more sensitive to the effects of power amplifier nonlinearity.
Compensating for the nonlinearity of an electronic component by predistortion is a technique which is known per se. It involves placing a component, hereafter denoted predistortion linearizer, upstream of the electronic component whose nonlinearity is to be corrected. The predistortion linearizer applies predistortion to the signal, which predistortion will then, at least in part, be corrected by the distortion of the electronic component. The predistortion parameters are selected so as to make the assembly (i.e. the predistortion linearizer and the electronic component whose nonlinearity is to be corrected) as linear as possible.
Hereafter, a distinction will be drawn between the electronic component (to be linearized) and the assembly comprising the electronic component and the linearizer, which will be denoted “linearized electronic component”. In the case of a (power) amplifier which is to be linearized, the assembly of linearizer and amplifier will therefore be denoted “linearized (power) amplifier”.
The advantage of placing the linearizer upstream of the amplifier rather than downstream of it resides in the fact that the electrical efficiency of the amplifier is less impaired.
FIG. 1 shows, qualitatively, the amplitude transfer curve (AM/AM curve representing output amplitude as a function of input amplitude) and the phase transfer curve (AM/PM curve representing the phase difference of the output signal as a function of input amplitude) of a traveling-wave tube power amplifier (TWTA) (without memory effect). It will be noted that a solid state amplifier does not exhibit saturation. Its AM/AM and AM/PM curves would therefore differ qualitatively from those shown in FIG. 1.
FIG. 2 shows the AM/AM and AM/PM curves of an ideal or ideally linearized amplifier. The AM/AM curve is linear up to saturation and flat thereafter, while the AM/PM function is constant. It will be noted that it is not possible to obtain a linear AM/AM curve extending beyond the saturation power of the amplifier. The linearized AM/AM curve has to be limited to this maximum power value. It will, on the other hand, be noted that the linearized gain, i.e. the gradient of the AM/AM curve, is, in principle, a free parameter. Gain should, however, be kept as high as possible so as not to reduce electrical efficiency. It is thus attempted to preserve to the greatest possible extent the saturation gain of the nonlinear amplifier.
The predistortion linearizer has an AM/AM curve which is the inverse (in terms of the mathematical functions: f1) of the AM/AM curve of the amplifier and an AM/PM curve, as a function of the output power thereof, which is the opposite of the AM/PM curve of the amplifier. The AM/AM and AM/PM curves of the linearizer are shown qualitatively in FIG. 3.
Linearization cannot extend beyond amplifier saturation (the inverse of the AM/AM curve no longer exists). The linearizer is therefore preferably complemented by a power limiter.
The theoretical AM/AM curve of the linearizer is not accurately physically achievable up to saturation. A second, more realistic, AM/AM curve is shown in FIG. 3. It is, however, necessary to get as close as possible to the theoretical AM/AM curve in the vicinity of saturation because the operating points close to saturation are those where electrical efficiency is most favorable. This point is less difficult on a solid state amplifier which does not have a saturation threshold.
The linearizer may be set:                once and for all at the start of operation of the linearizer-amplifier assembly. This variant has the disadvantage that variation in environmental conditions and drift over time of amplifier and linearizer characteristics may reduce linearization quality.        once and for all at the start of operation, but for a range of parameters such as temperature, frequency, gain, etc. in order to obtain a set of AM/AM and AM/PM curves, from which is selected the one which is best suited to the environmental conditions measured during operation of the linearized amplifier.        periodically, by one or more feedback loops in order to follow the variations, assumed to take place more slowly, in the environment and equipment characteristics.        permanently by one or more feedback loops.        
All previously known methods for calibrating predistortion linearizers directly measure the phase difference of the output signal relative to the input, either by using a laboratory instrument (network analyzer or vector voltmeter) at the start of operation, or by demodulating the output signal in order to obtain the channels I and Q (phase is then the argument of the complex number I+jQ) or alternatively the signal amplitude and phase.
Requirements for these calibration methods by phase measurement include:                having a receiver/demodulator which may be a laboratory measuring instrument or alternatively the receiver of the transceiver in the case of bidirectional telecommunications equipment;        defining specific measurement signals or alternatively using the signals, known in advance, which are already present in the useful signal such as synchronization pilots or rise and fall times;        defining a time and phase reference between the input and output signals and optionally calibrating said reference itself using specific signals.        
Document WO 2004/040751 A1 describes a linearizer which combines a number of linearization principles, namely predistortion linearization, feed-forward linearization and feedback linearization. A predistortion linearized amplifier is included in a feed-forward correction loop, the assembly being corrected or calibrated by a feedback loop.
Patent application US 2007/0190952 A1 discloses a mechanism for calibrating a digital predistortion linearizer. The mechanism uses a receiver integrated in the RF chip to demodulate the amplified RF signal at the amplifier output. The I and Q channels arising from demodulation are used to calculate the amplitude and phase of the amplified signal. Linearization is carried out in a processor upstream of the digital-analog conversion of the signal to be amplified. The amplitude of the signal is set by a code which directly activates the amplifier, while phase is processed by a different channel and used as an amplifier input signal.
Patent application US 2009/0058521 A1 describes a method for reducing the nonlinear distortion of an electronic component by applying a stimulus signal to the input of the electronic component and analyzing the distortion undergone by the stimulus signal. A correction signal is determined on the basis of the observed distortion and added to the primary signal upstream of the nonlinear electronic component. The stimulus signal is a bifrequency signal, the components of which have the same power. A plurality of distortion measurements are carried out in a first phase while maintaining a fixed separation between the frequencies of the two components and varying the central frequency and in a second phase while keeping the central frequency fixed but varying the separation between the frequencies of the two components.